1. Field of the Invention
The present invention relates to a method and to a device for carrying out operations of seismic monitoring of an underwater zone by means of elastic wave sources permanently installed against the bottom of the water mass.
2. Description of the Prior Art
During production of a reservoir, it is well-known to carry out cyclic seismic exploration operations with transmission in the formation of elastic waves, recording of the waves reflected by the subsoil discontinuities and processing of the acquired seismic data in order for example to follow the evolution of the reservoir with time.
A conventional method referred to as VSP consists in coupling the wall of a well to a string of seismic receivers placed at intervals along a conventional logging cable in order to pick up the waves reflected by the discontinuities of the surrounding formation in response to the waves emitted by a source outside the well.
In order to simplify and to accelerate these periodic seismic well exploration or monitoring sessions which require drilling operation interruptions for logging, it is well-known to install permanently one or preferably several sources with different offsets and/or azimuths in relation to the well, as described for example in patent French Patent-2,728,973.
Onshore, the source can be readily placed in contact with the ground or at the bottom of a cased hole deep enough for the source to be in contact with the formation below the weathered layer (WZ). Several independent sources coupled with the ground with different offsets in relation to the well can be successively activated for each position of the VSP logging tool.
Offshore, in order to carry out similar multi-offset, multi-azimuth VSP type exploration sessions, it is well-known to tow an immersed impulsive source (air, water or explosive gun, marine vibrator, etc.), to displace it in a zone around the well to a succession of "shooting" positions and to carry out, in each one of them, a succession of emission-reception cycles. It is clear that the duration of these interposed acquisition sessions is therefore considerably longer. It is faster and obviously much more expensive to use several workboats each towing a marine source and to trigger them in sequence for each position of the logging tool. Prospecting methods using towed immersed sources suffer from a common drawback. In fact, it is not possible to repeat exactly the same geometry of the emission-reception system from one exploration session to the next notably because of the ocean currents which contribute to changing the effective positions occupied by the source during the shots. The same uncertainty also exists for boats that are not equipped with dynamic positioning means allowing to stabilizing the position of the source.